Fig 1.
Different classes of fungicides (e.g., allyamines, azoles, and polyenes) inhibit pathogen growth by targeting the ergosterol biosynthesis pathway.
Allyamines and azoles inhibit key enzymes in ergosterol biosynthesis required for membrane function (A), while polyenes disrupt membrane function, leading to pore formation by binding to ergosterol (B). Different classes of plant sterol binding proteins targeting pathogen cell membranes are important components of plant defence. In return, fungal pathogens have evolved to overcome such defences. Avenacin disrupts membrane integrity by forming complexes with sterols. The enzyme avenacinase, secreted by the fungal pathogen Gaeumannomyces graminis var. tritici, degrades avenacin, a saponin-type sterol binding protein secreted by oat roots (C). The pathogenesis-related protein 1 (PR1) from tobacco binds membrane sterols through its C-terminal cysteine rich secretory protein/antigen 5/PR1 (CAP) domain and provides pathogen protection by sequestering sterols from membranes of the oomycete pathogens Phytophthora parasitica and Peronospora tabacina (D). A speculative model depicting how fungal wheat pathogens can potentially overcome PR1-mediated defences. Toxin (Tox) effectors (e.g., ToxA or SnTox3) secreted by Parastagonospora nodorum and Pyrenophora tritici var. repentis bind to the CAP domain of PR1, potentially (?) interfering with its sterol binding-mediated antifungal activity (E).